According to USC,
18 U.S. Code § 4. Misprision of felony
Whoever, having knowledge of the actual commission of a felony cognizable by a court of the United States, conceals and does not as soon as possible make known the same to some judge or other person in civil or military authority under the United States, shall be fined under this title or imprisoned not more than three years, or both.
Say if someone posted a post on SE that is clearly a crime under US federal laws. However it is unclear whether the author is of US citizenship. Is it a legal obligation for a US citizen to report such posts to authorities? Is it considered active concealing by down/close/delete voting the post?
My understandings: No. Since it is required that the case is cognizable by a US court, and it is not the case if the author is not a US citizen or on US territory; since it is unclear whether the author is a US citizen, one does not have knowledge of the actual commission of a felony cognizable by a court of the United States. Hence a US citizen is not legally obligated to report such posts, and down/close/delete voting is not a crime.